I think this is clearly a case of 用勁 being used in an ordinary sense of "exertion of force" rather than 勁 as a proprietary taiji term of art. I think the point being made in the Pressing Kick instruction is that it's not necessary to exert much force for the technique to be effective, especially if the opponent has lost his root. Just as a gentle An or Ji or whatever hand or arm movement can send an opponent away when applied at the right moment, the same holds true when applying the sole of the foot. When I was first taught the form, I was told that for the Separate Feet kicks, where the striking surface is the upper outer side of the foot, the objective is a sort of stinging slap to the sensitive flank, or the armpit, of an opponent. Then I was taught that the dengjiao kicks were meant to just make contact and kind of gently send the opponent off. The whole notion of severing the opponent's root and send him off (提放 tífàng) is neatly described in another part of the classic commentary in Brennan's Shiyongfa translation:

練十三勢要用柔法，然後功成就生出柔中含藏內勁，呼吸者，蓋吸能提得人起，能使敵後足離地，再呼氣力從脊內發出全身之勁放得人遠出，呼吸靈通，身法然後才能靈活無滯。When practicing the solo set, it should be done with softness. After you have worked at it for a long time, you will be generating internal power hidden within the softness. “Breathe” means that when you inhale you can lift [提] the opponent, making his rear foot leave the ground, then when you exhale, power comes from the spine, issuing with the power of the whole body, and sends [放] the opponent far away. By mastering breathing, your body’s techniques will then be quick and crisp.

I thought I’d share my translation of some analytical points from Yang Zhenji’s book regarding the kicks in taijiquan. I hope this adds some perspective on the issue raised in the Shiyongfa book: “When you do a pressing kick to someone, you must not use force.”~~~• The attacking movements of the Fen Jiao and Deng Jiao kicks in Yang Style Taijiquan without exception require standing straight on one leg, with a stable lower frame. To achieve a stable lower frame you must pay attention to the following: the straight standing leg must not be stiff, the torso must not draw up, and the qi should not float up; the lower form should possess a downward-sinking strength (下沉的力量). The classical taijiquan requirements, “an intangible and lively energy lifts the crown of the head,” “loosen the waist and qua,” “contain the chest and pull up the back,” “sink the qi to the dantian,” and “sink the shoulders and drop the elbows,” should all be implemented in these movements.

• The movement of the foot in both Fen Jiao and Deng Jiao is to lift the knee and employ the springy jin (彈勁) to kick out in an instant. It is not a use of the waist/kua jin (腰跨的勁), nor a use of strength from the thigh (大腿根也不用力). If you were to employ strength (用力) from the waist/kua, then it would change into a thrusting kick (chuai 踹 ), which is the foot method of other boxing styles. In contrast, according to the kicking method of Yang Style taijiquan, the use of strength from the waist/kua will cause your body to be uncentered, and the strength to dissipate and disintegrate. The characteristics of Yang Style taijiquan’s kicking method is completely in harmony with the idea expressed in the classics: “it is rooted in the feet, issued by the legs, governed by the waist, and expressed in the fingers. From the feet, to the legs, then to the waist, always there must be complete integration into one qi.” The foot that is kicking out is rooted in the other [standing] foot. The foot, waist, and kua link together to create a stable base. This is foundation for the other, kicking foot. Carefully ponder this method of using jin, and you will come to intuitively understand it in your mind.

（３）分、蹬脚等脚的动作是提膝用脚的弹劲一瞬间踢出，不是用腰胯的劲，大腿也不用力。如果腰胯用力，就会变成踹，是其他拳派的的脚法。按杨式太极拳的踢法用腰胯的力量反而使身不中正，力量分散不整。杨式太极拳踢法的特点与古典“其根在脚、发于腿、主宰于腰，形于手指，由腿而腿而腰，总须完整一气”的说法是相融的。踢出的脚的根在另一脚，脚、腰、胯相连坐稳，是踢出的另一脚的基础。细心揣摩这种用劲的方法，自然能体会于心。~~~Let me know what you think.

LEFT & RIGHT BRUSH KNEE IN A CROSSED STANCE 手揮琵琶 PLAY THE LUTE 進步搬攔錘 ADVANCE, PARRY, BLOCK, PUNCH

In this list and in the explanation of the postures there is no Brush Knee before Advance, Parry, Block, Punch.Is this an omission?Is this the sequence at the time of writing the book and an extra brush knee added later?

You're right, it's missing. The Left Brush Knee Twist Step was evidently left out inadvertently in the original book. It was in place in the later Essence and Applications book. Interestingly, both Shiyongfa and Essence and Applications leave out the third Wild Horse Parts Mane, needed for the transition into Grasp Sparrow's Tail. It strikes me as sort of a pragmatic thing, as though once a movement has been described, it's redundant to describe it again. Or it could merely have been editorial error in the original books.

LEFT & RIGHT BRUSH KNEE IN A CROSSED STANCE 手揮琵琶 PLAY THE LUTE 進步搬攔錘 ADVANCE, PARRY, BLOCK, PUNCH

In this list and in the explanation of the postures there is no Brush Knee before Advance, Parry, Block, Punch.Is this an omission?Is this the sequence at the time of writing the book and an extra brush knee added later?

Greetings BB,

In reading Paul Brennan's new translation of Chen Weiming's Taiji Dawen, I see now there is a better answer to your question regarding this sequence. Chen says that Yang Chengfu added in the final Brush Knee, but that previously he had left it out, doing the transition into Advance Step, Deflect Parry and Punch directly from Hands Strum Pipa. I'll post the section in the thread I started on Taiji Dawen.

I thought yang cheng-fu never authored any books. From my understanding this is why he told chen wei-ming to write them. This is why chen wei-ming wrote three books taiji quan shu tai chi ta wen and tai chi sword.

In 1925, Yang Cheng-Fu asked Chen Wei-Ming, to write a book entitled "Tai Chi Chuan", with detailed captions to Yang Cheng-Fu's pictures as illustrations. In 1931, Yang had all the pictures retaken and compiled into "The Methods of Taijiquan", which was revised two years later into "A Complete Book of Tai Chi Chuan".

Chen Wei-Ming was also allowed to author the book "Tai Chi Chuan Ta Wen", considered by most to be a modern classic in Tai Chi Chuan literature. This favorite pupil of Yang Cheng-Fu wrote three books on behalf of his teacher, whose desire it was to make Tai Chi Chuan more well-known to the public at large at the beginning of the last century. The third book written by Chen was "The Form of Tai Chi Chuan" (Taijiquan Shu, 1925).

"Tai Chi Sword and Other Writings" (1927) dedicates itself with texts and photos of the Tai Chi Sword form, as he learned from teacher Yang Cheng-Fu. The text is almost entirely a pure description of the individual movements of the sword form and probably originally served as a reminder to the pupils. We know, all too well, that even the most extensive texts and detailed designs and photos cannot replace a teacher, and thus the information in the available text is purely technical and contains nothing new about the form.

My teachers father learned under both yang cheng fu, and chen wei-ming and was one of chen wei-mings senior students and close friends. Thats the history as I know it. Please correct me if im wrong.

It was not uncommon for famous martial artists to commission the writing of their books to students who were capable and well-connected with those that had the means to publish books. In addition to the 1925 book published under Chen Weiming's name, two subsequent books were published under Yang Chengfu's name. One was compiled and written by Dong Yingjie, and later that book was edited and "polished" by Zheng Manqing. In both cases, the published books were based upon Yang Chengfu's demonstration narrative, so they indeed represent the true transmission of his teachings.